pixels-to-points

Arguments:
pixel-size

Returns a point size that is equivalent to the specified pixel size.
The argument and returned value may be any real number. There are
approximately 72 points in an inch, but since the actual monitor size
is unknown, the point size may not reflect reality very closely.

Also, in GTK there is a notion of a global "logical font
dots-per-inch" that can be arbitrarily different from what the
standard functions for dots-per-inch return. (Some window managers
allow the user to modify this value.) Before Pango 1.8 (typically
shipped with GTK 2.6), it appears impossible to determine the mystery
logical font DPI, and so pixels-to-points and points-to-pixels are based on the standard
dots-per-inch functions of GTK. Beginning in Pango 1.8, we can
deduce the logical font DPI, and so pixels-to-points and points-to-pixels are based on this value
instead. Therefore, these functions reflect the official point sizes
of fonts (where they are most useful) beginning with Pango 1.8
(typically in GTK 2.6) only. In earlier versions, this is why Common
Graphics fonts are often a different size than the requested pixel
size on GTK.